The present invention relates to a machine structural steel bar or wire having improved cold workability and used for producing machine structural parts such as automobile parts and construction machine parts, and process for producing the same.
Machine structural parts such as automobile parts and construction machine parts, for example, bolts, stabilizers or the like have heretofore been produced by cold forging a steel bar or wire made of a machine structural carbon steel or alloy steel.
That is, a machine structural carbon steel or alloy steel is generally hot-rolled. The rolled steel material is then softening-annealed for the purpose of ensuring cold workability, and finish wire-drawn for the purpose of increasing the dimensional accuracy and smoothing the surface. The resultant wire is then formed by cold working such as cold forging (e.g., thread rolling), and quench-tempered to give machine parts having a predetermined strength.
To produce a machine part such as a bolt, the softening annealing is effected by low temperature annealing to produce a stud bolt or the like with a small cold working amount, by normal annealing to produce a hexagon head bolt or the like, or by spheroidization annealing to produce a flange bolt or the like with a large cold working amount. As explained above, softening annealing is a heat treatment at high temperature for a long period of time; therefore, it not only reduces the productivity but also has a significant effect on the production cost from the standpoint of saving energy.
In order to diminish the load of softening annealing on the production, those parts which are to be subjected to cold working in a small amount are low temperature-annealed for a short period of time (about 5 hours) at the cost of softening degree. Only those parts which are to be subjected to cold working in a large amount are spheroidization-annealed for a long period of time (about 20 hours) so that the softening degree becomes the maximum value. When machine parts having a complicated shape are to be prepared by cold forging with a large cold working amount, parts for the machine parts must be softened to a sufficient degree by spheroidization annealing because surface defects and cracks are formed in the parts if the softening degree is insufficient.
When a steel bar or wire is to be formed into machine parts by cold working to have a predetermined shape, the steel bar or wire is typically cold forged with dies. For example, when a decrease in the strength of a steel material to be cold forged is 10 kgf/mm2 (softening), the life of the dies is improved by a factor of about 4 to 5.
It can be said, from the standpoints explained above, that the machine structural steel bar or wire is required to have a softening degree as high as possible by spheroidization annealing and that machine parts, having been formed by cold forging the softened steel material to have a predetermined shape, must be strengthened by a heat treatment such as quench tempering.
In order to meet the requirements explained above, various proposals have been made.
Japanese Unexamined Patent Publication (Kokai) No. 61-174322 proposes a method of softening a medium carbon structural steel in which pearlite transformation is finished in a short period of time and at a high temperature to soften the steel.
Japanese Unexamined Patent Publication (Kokai) No. 58-107146 proposes production of a steel bar or wire having improved cold forgeability and machinability in an as-hot-rolled state wherein a steel containing as basic components 0.10 to 0.50 wt % of C, 0.10 to 0.50 wt % of Si, 0.3 to 1.8 wt % of Mn and 0.0002 to 0.005 wt % of B is used, and the rolling conditions and the subsequent cooling conditions are restricted.
The conventional technologies proposed above improve the cold forgeability by softening the steel materials.
However, in order to further enhance the productivity, a machine structural steel bar or wire having a still higher softening degree and improved cold workability is demanded.
An object of the present invention is to provide a machine structural bar or wire having a high softening degree in comparison with a conventional spheroidization-annealed steel material, good hardenability and improved cold workability, and a process of producing the same.
In order to make the cold workability of a steel compatible with the hardenability, the present inventors have investigated a boron-containing steel having a low Si content, and they have found the novel results explained below.
When a low Si content boron-containing steel having a chemical composition in a selected range is subjected to low temperature rolling and slow cooling, special iron-boron-carbon carbides (borocarbides) are formed, and the steel has the following properties: (1) the fraction of pearlite is significantly decreased; (2) granular carbides are precipitated; and (3) a ferrite structure is markedly refined.
Next, when a steel material having the structure mentioned above is spheroidization-annealed, (1) the number of carbides per unit area is small, and the spacing of spheroidization-annealed carbides is wide; and (2) a structure in which matrix ferrite grains are fine is obtained. As a result, a steel bar or wire having a low strength, improved cold workability and excellent hardenability is obtained.
The present invention is based on the discoveries, and provides (1) to (12) described below.
That is, a first invention provides (1) to (4) described below.
(1) A machine structural steel bar or wire excellent in cold workability,
comprising 0.1 to 0.5 wt % of C, 0.01 to 0.15 wt % of Si, 0.2 to 1.7 wt % of Mn, 0.0005 to 0.05 wt % of Al, 0.005 to 0.07 wt % of Ti, 0.0003 to 0.007 wt % of B, 0.002 to 0.02 wt % of N and the balance of Fe and unavoidable impurities, the unavoidable impurities including up to 0.02 wt % of P and up to 0.003 wt % of O, and
having a microstructure comprising ferrite and spheroidal carbides, the ferrite having a ferritic grain size number of at least No. 8 and the number of the spheroidal carbides per unit area mm2 being up to 1.5xc3x97106xc3x97C wt %.
(2) The steel bar or wire according to (1) described above, wherein the steel bar or wire further comprises 0.003 to 0.15 wt % of S.
(3) The steel bar or wire according to (1) or (2) described above, wherein the steel bar or wire further comprises up to 0.8 wt % of Cr, and the total content of Mn and Cr is from 0.3 to 1.3 wt %.
(4) The steel bar or wire according to any one of (1) to (3) described above, wherein the number of spheroidal carbides per unit area mm2 is up to 4xc3x97105xc3x97C wt %.
In order to produce the steel bar or wire of the first invention, a second invention provides (5) to (8) described below.
(5) A process of producing a machine structural steel bar or wire excellent in cold workability, comprising the steps of: hot rolling a steel comprising 0.1 to 0.5 wt % of C, 0.01 to 0.15 wt % of Si, 0.2 to 1.7 wt % of Mn, 0.0005 to 0.05 wt % of Al, 0.005 to 0.07 wt % of Ti, 0.0003 to 0.007 wt % of B, 0.002 to 0.02 wt % of N and the balance of Fe and unavoidable impurities, the unavoidable impurities including up to 0.02 wt % of P and up to 0.003 wt % of O, while the steel material surface is held at temperatures of Ar3 to Ar3+150xc2x0 C. on the outlet side of final finish rolling; cooling the hot rolled steel material at a rate up to 0.7xc2x0 C./sec in the temperature range from finish rolling temperature to 600xc2x0 C., whereby the steel material cooled to room temperature has a structure which comprises ferrite, lamellar pearlite and granular carbides, the fraction in terms of area ratio of lamellar pearlite being up to 90xc3x97C wt %, and the ferritic grain size number according to JIS G0552 of the ferrite being at least No. 9; and spheroidization-annealing the steel material.
(6) The process according to (5) described above, wherein the steel further comprises 0.003 to 0.15 wt % of S.
(7) The process according to (5) or (6) described above, wherein the steel further comprises up to 0.8 wt % of Cr, and the total content of Mn and Cr is 0.3 to 1.3 wt %.
(8) The process according to any one of (5) to (7) described above, wherein the hot rolled steel material is cooled at a rate up to 0.3xc2x0 C./sec in the temperature range from finish rolling temperature to 650xc2x0 C., and the fraction in terms of area ratio of the lamellar pearlite is up to 65xc3x97C wt %.
(9) The steel bar or wire according to (1) or (2) described above, wherein the steel bar or wire further comprises at least one element selected from the group consisting of up to 1.5 wt % of Cr, up to 3.5 wt % of Ni, up to 1.0 wt % of Mo, 0.005 to 0.1 wt % of Nb and 0.03 to 0.4 wt % of V, and the number of spheroidal carbides per unit area mm2 is up to 7.5xc3x97106xc3x97C wt %.
(10) The steel bar or wire according to (9) described above, wherein the number of spheroidal carbides per unit area mm2 is up to 2xc3x97106xc3x97C wt %.
(11) The process according to (5) or (6) described above, wherein the steel further comprises at least one element selected from the group consisting of up to 1.5 wt % of Cr, up to 3.5 wt % of Ni, up to 1.0 wt % of Mo, 0.005 to 0.1 wt % of Nb and 0.03 to 0.4 wt % of V, and the fraction in terms of area ratio of the lamellar pearlite is up to 170xc3x97C wt %.
(12) The process according to (11) described above, wherein the hot rolled steel material is cooled at a rate up to 0.3xc2x0 C./sec in the temperature range from finish rolling temperature to 650xc2x0 C., and the fraction in terms of area ratio of lamellar pearlite is up to 120xc3x97C wt %.